1. Field of the Invention
The present invention relates to a surface acoustic wave device having a structure in which a surface acoustic wave element is bonded to a substrate by a face-down mounting method, and more specifically, the present invention relates to a surface acoustic wave device having an improved structure including a substrate with a surface acoustic wave element laminated thereon.
2. Description of the Related Art
Miniaturization and low profile of a surface acoustic wave device accommodating a surface acoustic wave element has been strongly required of surface acoustic wave devices. To meet these requirements, a structure in which the surface acoustic wave element is bonded on the substrate via a bump, with a surface having an electrode for the surface acoustic wave element such as an IDT formed thereon defining a lower surface, i.e., a structure in which the surface acoustic wave element is mounted on the substrate by a so-called face-down mounting method, has been proposed.
In addition, the surface acoustic wave device accommodating the surface acoustic wave element and having an airtight or liquid-tight sealing structure has been required so as to stabilize the characteristics thereof, and so as to be less susceptible to fluctuations in the environment.
Japanese Unexamined Patent Application Publication No. 8-265096 discloses an example of the surface acoustic wave device of this type. In this example, the surface acoustic wave element is mounted on the substrate via the bump by the face-down mounting method, and, in order to seal the surface acoustic wave element, a metal cap is fixed to an upper surface of the substrate so as to surround the surface acoustic wave element. In addition, the surface acoustic wave element is bonded to the substrate also by insulating resin, and the bonding strength of the surface acoustic wave element to the substrate is increased. In order to prevent any flow casting of this insulating resin to the electrode side of the surface acoustic wave element, a guard layer is disposed on the upper surface of the substrate outside the surface acoustic wave element.
However, in the device disclosed in this Japanese patent publication, because the metal cap must be bonded to the substrate by using adhesive, etc. as described above, the number of parts is increased, and the assembly is complicated.
On the other hand, another structure has been known, in which the surface acoustic wave element is sealed in an airtight or liquid-tight manner by mounting the surface acoustic wave element on the substrate by the face-down mounting method, and sealing the surface acoustic wave element with resin except for a surface of the surface acoustic wave element having the electrode disposed thereon. In such a device, a metal cap need not be prepared separately, and an assembly process can be simplified.
FIG. 7 is a sectional view showing an example of a conventional surface acoustic wave device having a resin sealing layer. In a surface acoustic wave device 101, a surface acoustic wave element 103 is mounted on a substrate 102, and a resin sealing layer 104 is arranged so as to surround the surface acoustic wave element 103. Electrode lands 102a and 102b are disposed on an upper surface of the substrate 102. Via hole electrodes 102c and 102d are formed in the substrate 102 in a piercing manner through the substrate 102. Upper ends of the via hole electrodes 102c and 102d are connected to the electrode lands 102a and 102b. In addition, mounting electrodes 102e and 102f are disposed on a lower surface of the substrate 102 so that the surface acoustic wave device 101 is mounted on a printed circuit board, etc. A shield ring 102g is disposed on the substrate 102 so as to provide an electromagnetic shield effect.
On the other hand, a schematically shown electrode 103a for the surface acoustic wave element is disposed on a lower surface of the surface acoustic wave element 103. Bump pads 103b and 103c on the lower surface of the surface acoustic wave element 103 and the electrode lands 102a and 102b are connected to each other via metal bumps 105 and 106.
In the surface acoustic wave device 101, mounting electrodes 102e and 102f are disposed on the lower surface of the substrate 102. The surface acoustic wave device 101 is surface-mounted on a printed circuit board or other substrate by using the mounting electrodes 102e and 102f. 
FIG. 8 is a sectional view of another example of a conventional surface acoustic wave device of this type.
In a surface acoustic wave device 111 shown in FIG. 8, unlike the surface acoustic wave device 101, a substrate with the surface acoustic wave element 103 mounted thereon is a multi-layered substrate 112. This means that the multi-layered substrate 112 has electrode lands 112a and 112b on an upper surface thereof in a manner similar to the substrate 102. Via hole electrodes 112c and 112d are formed within the multi-layered substrate 112. However, via hole electrodes 112c and 112d are formed in the multi-layered substrate 112 so that lower ends of the via hole electrodes 112c and 112d reach the position of the middle height of the substrate 112, and connected to internal electrode 112e and 112f. Via hole electrodes 112g and 112h are formed on lower surfaces of the internal electrodes 112e and 112f. The distance between the via hole electrodes 112g and 112h is larger than the distance between the via hole electrodes 112a and 112b. 
In addition, the via hole electrodes 112g and 112h are arranged so as to reach the lower surface of the substrate 112, and connected to mounting electrodes 112i and 112j disposed on the lower surface thereof.
FIG. 9 is a sectional view of yet another example of a conventional surface acoustic wave device of this type. In a surface acoustic wave device 121, electrode lands 122a and 122b are disposed on an upper surface of a substrate 122, and mounting electrodes 122c and 122d are disposed on a lower surface thereof. The electrode lands 122a and 122b on the upper surface and the mounting electrodes 122c and 122d on the lower surface are electrically connected to each other via the end surface electrodes 122e and 122f disposed on the other side of the substrate 122.
In order to reduce the size and the cost of a surface acoustic wave device, it is necessary to obtain as many surface acoustic wave elements as possible from one wafer. Thus, the cutting margin when cutting individual surface acoustic wave elements out of the wafer is reduced, or the size of the surface acoustic wave elements themselves is reduced.
When the size of the surface acoustic wave elements is reduced, the distance between bump pads for bonding the surface acoustic wave elements via bumps is reduced. This means that surface bump pads 131a to 131d are disposed in a surface acoustic wave element 131 shown in FIG. 10. Electrodes for surface acoustic wave elements such as IDTs are also formed on a surface with the bump pads 131a to 131d disposed thereon although this is not shown in FIG. 10.
It is assumed that the surface acoustic wave element 131 is square in plan view, the length of one side is a, and the distance between bump pads is c. It is assumed that the size of the surface acoustic wave element 131 is reduced to obtain a surface acoustic wave element 132 shown in FIG. 11. In this occasion, the length b of one side of the surface acoustic wave element 132 is smaller than the length a, and similarly, the distance d between the bump pads 132a to 132d is liable to be shorter than the distance c shown in FIG. 10.
As described above, if the size of the surface acoustic wave element is reduced, the distance between the bumps must be smaller when the surface acoustic wave element is bonded on the substrate via bumps by the face-down mounting method. For example, if the size of the surface acoustic wave element 101 shown in FIG. 7 is reduced, a surface acoustic wave device 141 shown in FIG. 12 is obtained, and the distance d between bumps in the surface acoustic wave device 141 must be smaller than the distance between the bumps 105 and 106 in the surface acoustic wave device 101 shown in FIG. 7. Thus, in the surface acoustic wave device 141, the distance between the mounting electrodes 102e and 102f is reduced, and a short circuit can occur during the mounting.
On the other hand, in the surface acoustic wave device 111 including the multi-layered substrate 112 shown in FIG. 8, the distance between the mounting electrodes 112e and 112f can be sufficiently large by setting the distance between the via hole electrodes 112g and 112h therebelow to be larger than the distance between the via hole electrodes 112c and 112d even when the size of the surface acoustic wave element 103 is reduced. However, in the surface acoustic wave device 111, the mounting electrodes 112e and 112f are disposed on the lower surface of the multi-layered substrate 112, and any fillet formation in a solder-bonded portion to the printed circuit board or substrate cannot be checked. Therefore, defective mounting cannot be discovered rapidly.
On the other hand, in the surface acoustic wave device 121 shown in FIG. 9, the electrode lands 122a and 122b on an upper surface of the substrate 122 and the mounting electrodes 122c and 122d on the lower surface of the substrate are electrically connected to each other by using the end surface electrodes 122e and 122f. Thus, any fillet formation at an end surface can be visually checked when the surface acoustic wave device 121 is soldered to the printed circuit board or substrate.
However, since the electrode lands 122a and 122b are extended to the exterior of the resin sealing layer 104, and bonded to the end surface electrodes 122e and 122f, any shield ring or the like defining an electromagnetic shield structure cannot be disposed on the upper surface of the substrate 122. In addition, a problem occurs in that the sealability of the interface between the resin sealing layer 104 and the substrate 122 is degraded.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device which solves the above-described problems of the prior art, and has a surface acoustic wave element mounted on a substrate by a face-down mounting method, can be easily surface-mounted on a printed circuit board or other substrate even when the surface acoustic wave device has a resin sealing layer, has a greatly reduced size of the surface acoustic wave element, is constructed to facilitate easy checking of any fillet formation, prevents any generation of a short circuit or other such defect and disadvantage, and increases versatility in wiring an electromagnetic shield ring or the like.
A surface acoustic wave device according to a preferred embodiment of the present invention includes a surface acoustic wave element having first and second main surfaces facing each other and a plurality of end surfaces connecting the first and second main surfaces, and an electrode for the surface acoustic wave element and a bump pad for electric connection to the exterior on the first main surface, and a substrate having an electrode land joined with the bump pad of the surface acoustic wave element via a bump, and a resin sealing layer arranged so as to cover the surface acoustic wave element such that the surface acoustic wave element is joined with an upper surface of the substrate from the first main surface side via the bump, and the surface acoustic wave element is spaced from the upper surface of the substrate, the substrate includes a multi-layered substrate, the multi-layered substrate includes a via hole electrode in which one end thereof is connected to the electrode land provided on the upper surface of the multi-layered substrate, an internal electrode connected to the via hole electrode, a mounting electrode provided on a lower surface of the multi-layered substrate, and an end surface wiring electrode with which the electrode for mounting and the internal electrode are connected at an end surface of the multi-layered substrate.
According to another preferred embodiment of the present invention, the surface acoustic wave device further includes a protective layer which is disposed on the first main surface of the surface acoustic wave element to protect the electrode for the surface acoustic wave element and a frame member which surrounds a portion including the electrode for the surface acoustic wave element to prevent flow of a resin constituting the resin sealing layer.
According to yet another preferred embodiment of the present invention, the sealing resin is a thermosetting resin or a photosensitive resin.
According to still another preferred embodiment of the present invention, the multi-layered substrate is preferably made of ceramic or synthetic resin, or other suitable material.
According to further still another preferred embodiment of the present invention, a shield ring is provided on the upper surface of the multi-layered substrate so as to surround a portion on which surface acoustic wave element is mounted.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.